1. Field of the Invention
This invention relates to ink compositions suitable for ink jet printing.
2. Background Art
Ink jet printing is a non-impact printing process in which the printer produces droplets of ink in response to digital signals, such as those generated by a computer. The droplets of ink are deposited on a substrate such as paper or transparent films. Ink jet printers have found broad commercial acceptance due to their print quality, low cost, relatively quiet operation, and graphics capability. Thermal (bubble jet) and piezoelectric drop-on-demand printers have been especially successful in the marketplace and have found broad application as printers for personal computers in the office and the home.
Three major concerns dominate ink jet technology: (1) reliability, (2) print quality, and (3) drying rate. Typically, reliability is evaluated with respect to the following four criteria. The first is robustness with respect to continuous printing conditions such that the ink droplet weight does not change over time and good directionality is maintained. Good directionality means that the angular deviation of an ejected ink droplet from a nozzle is within about .+-.0.5.degree. from the normal to the plane of the nozzle. The second is robustness with respect to intermittent printing conditions such that the nozzles do not clog over the time interval in which printing is discontinued. The third is robustness with respect to long term storage of the ink within the print head such that printing behavior unchanged from the original printing behavior (ink droplet weight and good directionality) can be restored after applying a limited amount of suction to the nozzles. The fourth is chemical and physical stability of the ink towards storage at two temperature extremes and towards cycling between those two temperature extremes for an extended period of time.
Print quality is typically defined in terms of two general factors: color properties and non-color image characteristics. Color properties of the ink are measured by optical density and the color coordinates which determine the hue. Non-color characteristics which determine the definition of an image are resolution (drops per unit area), the area coverage per drop, edge acuity or sharpness, and the extent of peripheral defects such as satellites (stray droplets around the perimeter of a printed character) or feathering.
A great concern with ink jet printing is the level of print quality, as defined by edge acuity or sharpness of an image and minimal feathering, which can be obtained on "plain paper." The term "plain paper" refers to a broad set of commercial papers, especially those available for use in electrographic copying. Such commercial papers do not rely on a unique structure, composition, or narrow set of properties such that only an ink jet printer can make best use of the paper's properties. In recent years there has been increasing demand for ink jet printers which provide excellent print quality on plain paper.
The drying rate of the ink is an important factor in determining the throughput rate of the printer. In a sheet fed printer, the ink on a printed sheet must be dry before the succeeding sheet contacts it. If the ink is not dry, smearing will occur. Drying occurs by both evaporation and penetration. The drying rate can be increased by using a water miscible cosolvent, which has a higher vapor pressure than water, and, thus, will evaporate faster than water. This approach may be disadvantageous because the cosolvent will also evaporate more rapidly from the nozzles of the print head, resulting in clogging during periods of non-use. The preferred approach is to use a water miscible cosolvent which increases the rate of penetration of the ink into the paper. The use of penetrants such as diethylene glycol mono-n-butyl ether, which lower the interfacial energy between the ink and the paper, is well known in the art.
The three major concerns discussed above are often incompatible. For example, as noted above, increasing the drying rate by using a cosolvent which has a higher vapor pressure than water, will adversely affect the reliability of the ink. Similarly, the use of penetrants is known to adversely affect the print quality of the ink on plain papers. Accordingly, there exists a need for ink compositions which will give reliable printing performance, dry rapidly, and yield printed images which have excellent print quality on plain paper.
One general approach which has been used to obtain improved print quality on plain paper is to include in the ink a gel-forming polymer which will gel when it comes in contact with a gel-initiating species. The formation of a gel results in reduced spreading of the ink droplet on the surface of the paper. Reduced spreading of the ink droplet leads to better edge acuity and reduced feathering.
In one method using this concept, the gel-initiating species is dissolved in a clear, dye-free liquid and then printed on the paper prior to the printing of the inks which contain the gel-forming polymer. In another method using this concept, one color of ink contains the gel-initiating species and another color of ink contains the gel-forming species. Using this method, significant improvements in print quality are only obtained when the two inks are printed on top of each other or adjacent to each other. In the ideal method using this concept, a gel-forming polymer is used which is capable of forming a gel in the presence of gel-initiating species which are contained in the compositions of typical plain papers. Examples of prior art which use the gel-forming polymer and gel-initiating species concept are given below.
In Japan Laid-Open Patent Application 01-203,483, an ink composition containing a water soluble dye, a polyhydric alcohol derivative, and pectin in the amount of 0.01 to 2 wt. % is described. The claimed advantages of the ink compositions are rapid drying and printed images on plain paper having good print quality. Additionally, good reliability of the ink and lightfast and waterfast print images are claimed.
In Japan Laid-Open Patent Application 04-190947, an ink set is described in which a first liquid containing a water soluble resin having at least one carboxylic acid group, carboxylic acid anhydride, or a sulfonic acid group is applied to a substrate followed by a second aqueous ink composition which contains a dye, an alkali-soluble resin, and an alkali. The pH of the first liquid is between 1 and 7, while the pH of the second liquid is between 7 and 12. Pectinic acid (a synonym for pectin) is used in one example of the invention as the carboxylic acid group containing water soluble resin of the first liquid. As an example of an alkali-soluble resin for use in the aqueous ink composition, pectin is given. The claimed advantage of the ink set is printed images having good print quality. The pH of the alkali-soluble resin contained in the ink is lowered when it comes in contact with the acidic resin contained in the first liquid which is deposited on the substrate. Lowering of the pH of the alkali-soluble resin results in deposition of that resin and, thus, limited spreading of the dye. When contiguous blocks of yellow, magenta, cyan, and black inks are printed on top of the first liquid, neither color bleeding not uneven mixing were observed at the boundary areas between contiguously printed inks.
In Japan Laid Open Patent Application 04-233693, an ink jet recording process is described with a first ink having a pH in the range from 1 to 6, a second ink having a pH in the range from 4 to 8, a third ink having a pH in the range from 6 to 10, and a fourth ink having a pH in the range from 8 to 13. The first ink contains a water soluble resin having at least one carboxylic acid group, carboxylic acid anhydride, or a sulfonic acid group. The second ink contains a water soluble resin with the same functionalities as that of the first ink or an alkali-soluble resin. The third and fourth inks contain an alkali-soluble resin. Pectinic acid (a synonym for pectin) is used in one example of the invention as the carboxylic acid group containing water soluble resin of the first or second inks. As an example of an alkali-soluble resin for use in the second, third and fourth inks, pectin is given. The claimed advantage of the ink set is printed images having good print quality. With the pH's of the four inks differing as described above, mixing of any two inks results in at least partial deposition of the resins or an increase in at least one of the ink's viscosity. This resin deposition or increased ink viscosity results in limited spreading of the dye(s). When contiguous blocks of yellow, magenta, cyan, and black inks are printed, neither color bleeding not uneven mixing were observed at the boundary areas between contiguously printed inks.
In Japan Laid Open Patent Application 04-348639, an ink jet recording process is described with the yellow, magenta and cyan inks containing a water soluble resin having at least one carboxylic acid group, carboxylic acid anhydride, or a sulfonic acid group and the black ink containing an alkali-soluble resin. The pH's of the yellow, magenta and cyan inks are lower than that of the black ink. The reverse combination is also described. Pectinic acid (a synonym for pectin) is used in one example of the invention as the carboxylic acid group containing water soluble resin (yellow ink). As an example of an alkali-soluble resin, pectin is given. The claimed advantage of the ink set is printed images having good print quality. With the pH of the black ink differing from the pH's of the yellow, magenta and cyan inks as described above, mixing of the black ink with the yellow, magenta and cyan inks results in at least partial deposition of the resins or an increase in at least one of the ink's viscosity. This resin deposition or increased ink viscosity results in limited spreading of the dye(s). When black blocks are printed contiguous to blocks of yellow, magenta, cyan, neither color bleeding not uneven mixing were observed at the boundary areas between the contiguously printed inks.
In U.S. Pat. No. 5,476,540, a method for controlling color bleed between adjacent multicolor regions on a print medium is described. A first composition containing a gel-forming species and a colorant is brought into contact on a region of the print medium with a second composition having a gel-initiating agent and a colorant. In an alternative embodiment of this invention, the print medium may be pretreated with either a gel-initiating or gel-forming species (with no colorant), followed by treatment with a gel-forming species or gel-initiating species (with colorant), respectively. Gel-forming species are selected from a group which includes pectin. Gel-initiating species are selected from a group which includes mineral acids, organic acids, trimethylammonium chloride and ions selected from the group consisting of calcium, magnesium, aluminum, and iron. The formation of the gel upon the print medium impedes the movement of the colorant(s) and, thus, results in improved print quality.
In United States Patent No. 5,112,339, inks for printing on plain paper which comprise a vehicle containing 5 to 20 wt. % of a glycol or glycol ether, 1 to 6 wt. % of a dye, and 0.1 to 1 wt. % of an alginate are described.
In U.S. Pat. No. 5,133,803, ink jet inks which contain high molecular weight colloids, such as alginates, and amphoteric surfactants, and/or non-ionic surfactants are described. The high molecular weight colloids are selected from a group consisting of alginic acid, mannomuronic acid, carrageenan, guar gum, xanthan gum, dextran, chitin, chitosan, and carboxymethylcellulose. The presence of the high molecular weight colloid is claimed to improve the print quality.
All of the above examples represent significant advances in ink jet ink technology, especially with regard to improved print quality. However, there remains a need for ink compositions which can provide further improvements in print quality.